Line finder control circuit for communication systems



Dec. 27, 1949 P. R. ADAMS T AL 2,492,344

. LINE FINDER CONTROL CIRCUIT FOR COMMUNICATION SYSTEMS Filed Nov. 14,1945 '7 Sheets$heet l I FIG-1 COMMON EQUIPMENT TO OTHER LINKS SECONDLINK CIRCUIT LIN REG SELEC RCUIT CIRCUIT FIRST LINK CIRCUIT FIG. 2

FROM POWER SUPPLY 7 Sheets-Sheet 2 P. R. ADAMS ET AL LINE FINDER CONTROLCIRCUIT FOR COMMUNICATION SYSTEMS mmnzIm INVENTORS PAUL R. 40,4445 DAV/D H. FHA/50M ATTORNEY Dec. 27, 1949 Filed Nov. 14, 1945 Dec. 27, 1949P, R, ADAMS ET AL 2,492,344

LINE FINDER CONTROL CIRCUIT FOR COMMUNICATION SYSTEMS Dec. 27, 1949Filed NOV. 14, 1945 P R. ADAMS ET AL LINE FINDER CONTROL CIRCUIT '7Sheets-Sheet 4 :5 1 J IO KC PEAKED CLIPPER LOCK-IN 0s AMPLIFIERAMPLIFIER l0 KC +o-o21. x PHASE CORRECTOR A TTORNE) Dec. 27, 1949 FiledNov. 14, 1945 P. R. ADAMS ET AL LINE FINDER CONTROL CIRCUIT FORCOMMUNICATION SYSTEMS '7 Sheets-Sheet 5 vvvv CIRCUIT SECOND REGISTERCIRCUIT REGISTER CIRCUIT FOURTH REGISTER CIRCUIT i FlF-TH I28 REGISTERINVENTOR5 PAUL R AO/I/ IS fl/lV/D H. FAA 50M A TTOPNEV Dec. 27, 1949Filed Nov. 14, 1945 P. R. ADAMS ET AL LINE FINDER CONTROL CIRCUIT FORCOMMUNICATION SYSTEMS I '7 Sheets-Sheet 6 :42 44 --T L I Q! I r----"-'--[so I I I I I I I 5 MS I DELAY LINEI I I48 l I I l FIRST z I DELAY IIRsT ZERO GATE J I GATE I I C I 5+ l i I I l I 1 l l I54 I v 8+ I- THIRDI I44 ZERO GATE IDELAY GATE I I 50 MS LA I34 E Y 155 FOURTH FOU RTH |s|I35 I45 ZERO GATE IDELAY GATEI 40 MS I36 DELAY J FIFTH v FIFTH I53 I 6ZERO GATE DELAY GATE :1 so MS DELAY INVENTORS ATTORNEY 1949 P. R. ADAMSET AL 2,492,344

LINE FINDER CONTROL CIRCUIT I 7 FOR COMMUNICATION SYSTEMS Filed Nov. 14,1945 7 Sheets-Sheet 7 IOA E89 T98 H-69 Patented Dec. 2 7,v 1949 lllTEDSTATES TEN OFFICE LINE FENDER CONTROL CIRCUIT FOB COMMUNICATION SYSTEMSApplication November 14, 1945, Serial No. 628,612

This invention relates to separation circuits 1 and more particularly toseparation or dividing pulses according to their signal carrying andother functions for use in telephone exchange systems.

In telephone systems generally in use, interconnection betweensubscribers lines through the various trunking lines in telephoneexchange requires considerable mechanical switching and a large plantset up. Furthermore, large numbers of interconnecting lines aregenerally required in the exchange so that connection may be madebetween any two lines incoming into the system. Likewise, considerablecomplication is presented in the signalling system for line selectionsand ringing.

Some replacement of mechanical switching systems by electronic switcheshave been proposed but, in general, all of these systems require stillthe mechanical selection of lines for interconnection. The electronicswitches as proposed generally are used simply to replace some of themechanical switches in the system. Furthermore, ringing and othersignalling is carried through conventional switching circuits in thesame manner as in the telephone systems generally in use.

It is an object of our invention to provide a switching circuit forinterconnection of channels wherein signal pulses may be used toestablish and maintain connections and to carry the signals through thesystem.

It is a further object of our invention to provide a system fordistributing an incoming signal in the form of pulses, and to use thesepulses for establishing and maintaining the complete communicationconnections.

It is a still further object of our invention to provide means forproducing pulses, modulated less that 70% in amplitude for conveyinginformation, and using the lower unmodulated portions of these pulsesfor establishing a circuit connection, while using the modulatedportions of these pulses for conveying the signals through suchconnections.

It is a still further object of our invention to provide a pulseseparating or dividing circuit wherein the lower unmodulated portion ofa series of pulses forming a pulse train is obtained in one outputcircuit, and another series of pulses varying in amplitude in accordancewith signals isobtained in a second output circuit.

According to a feature of our invention, a train of pulses for eachchannel of communication is established. These pulse trains aredifferently 8 Claims. (Cl. 179-18) timed so as to be successivelyeffective. The pulses are preferably given such a bias that theycontain, a substantial amplitude component even when fully amplitudemodulated. The percentage modulation is maintained under 70%. andpreferably under The pulses of each train 'serve to operate selectorcircuits, such as telephone line finders and simultaneously to carry thesignals and communication energy. To perform these two separatefunctions, the modulated pulses are clipped at a level below and abovethe modulation envelope. The constant amplitude pulses below themodulation level are usedto establish and maintain the line finderconnections to complete a circuit for transfer of the signals andcommunication.

T The divisionfor clipping of the pulses is best accomplished by asingletube circuit provided with a cathode resistor output and a normal anodeoutput. The pulses are applied to the grid of this tube. Controlpotentials are provided such that the cathode resistor output will passonly those portions of the applied pulses below a given level,preferably such that none of the modulation amplitude variation ispassed. The control potentials to the tube also assure that only theupper laortionjs of the pulses will appear in an anode output. limitedsothat the effective percentage modulation is high, up to 100% at thehighest signal levels. The constant amplitude pulse portions servecontinuously to maintain connections for the variable amplitude portionsto pass. Because of the systemused in selection and maintaining of theconnections, it is desirable to have these pulses of equal amplitude atall times.

In a telephone system incorporating our invention, signal or speechcurrents in the various lines or other channels may be replaced at theexchange by a series of narrow pulses of amplitude corresponding to theamplitude of the original current at the corresponding time. The pulsesare producedat suiiicient rapidity so that they define substantially thesignal envelope. In this manner by allotting different time positions toeach line, the signal or voice currents within the exchange may bedistributed over a common channel. each signal being repeated by aseries of pulses displaced in time in accordance with theldistributortime position. This distribution may be readily accomplished by means ofa cathode ray tube serving as a distributor which will sequentially scanthe lines connected to predetermined terminalsand respond if there is asignalling voltage on the line. The channels Preferably, the anodeoutput is may be separated by time selection and may be applied throughtime displacement means and a low-pass filter which serves to reproducethe audio envelope to the same or another distributor also coupled tothe lines. The incoming signals may serve to adjust the timedisplacement means so that they will represent the time differencebetween the time position of the calling line and the selected calledline. The time displacement means may be an actual delay line of someform or an equivalent circuit which, while not pro ducing an actualdelay of the signals, will 'efiectively serve to store the energy andrelease it after a predetermined interval equal to the desired delay. Inthis manner, the interconnection of any one line with any other line ofthe system may be accomplished. Upon making this interconnection, thecommunication signals may pass through the same delay means between theinterconnected lines. Furthermore, since the scanning cycle covers eachof the lines connected to the distributor, as many simultaneousconnections may be made as there are time displacement trunking channelswithin the exchange.

Preferably, means are provided responsive to the interconnection of thelines to tie up these lines so that they cannot be selected by anothersubscriber attempting to get the connection. If desired, anyconventional type of busy signal may be applied to the subscrib'ers linewhen this 'condition exists so that he will know that he must wait aninterval for the line to become free so that he can make the desiredconnection.

While we have broadly outlined certain objects and features of ourinvention, a better under standing of our invention and the objects andfeatures thereof may be had from the particular description of anembodiment of a telephone exchange incorporating our invention made withreference to the accompanying drawings, in which:

Fig. l is a block'diagram illustrating the gen eral circuit set up; D

Figs. 2 and 3 are sectional circuit diagrams and views respectively, ofa'distributor tube used in our system;

Figs. 4 to 8 inclusive, constitute a circuit diagram of a link exchangein accordance with our invention;

Fig. 4 illustrating the common equipment,

Fig. 5 showing the pulse forming-equipment,

Fig. 6 the line finder equipment, 7

Fig. 7 the dial register equipment, and

Fig. 8 the line selecting e'quip'mentt Fig. 9 is a diagram illustratinghow Figs. 4 "to 8 inclusive, should be arranged to illustrate thecomplete circuit; V

Fig. 10 is a set of curves used explaining the operation of certainparts or the system; and

Fig 11 is a diagram in section of a delay line suitable for use in theequipment shown in Fig. 8.

In the example outlined above, the system may be divided into threeparts "as shown in Fig. l: first, all the subscribers lines, twenty forex ample, assigned numerals -1 to-20, each-of these lines having asubscriber sub-set equipment such as 21; second, the equipment common toall line circuits, hereafter referred to asfconir'non e'quipment 22; andthird, a group of link circuits one 'of which is needed for eachsimultaneous call. Each of the link circuits may be'further subdividedinto line finder circuit 23, dial pulse forming circuit 24, dialregister circuit 25 and line selecting circuit 26. These several majorcomponents are interconnected by wires 21-38 inclusive, as shown inFig. 1. For the sake of simpilicity in the description only one-wayconversation is provided for.

As shown, all lines I to 28 terminate in common equipment 22. Thisequipment 22 performs a scanning function, preferably by means of asuitable tube having an electronic beam which sweeps each of the linesin turn.

7 When one of these lines has a potential indicative of a callingcondition, the common equipment 22 applies signals over wires Ti and 28to all the link circuits in parallel and specifically to the line findercircuit 23 of the first link (chosen for discussion) This line finder 23operates to find the calling line and transfer the signals over wire 33to the dial pulse forming circuit 24.

When dialing ensues, this circuit 24 produces dial pulses which arecounted and stored in dial register circuit 25. The dial pulse register25 then serves to control the line selector circuit 26 which maycomprise a delay line or other time displacement apparatus.

The incoming speech signals are then transferred from common equipment22 over 'wire 28, line finder circuit 23, wire 33, line selector-circi'iit '26 and thence over wire 36 back to the common equipment 22,from whence they are applied to the selected outgoing line. The part ofFig. 1 comprising line finder '23, dial pulse forming circuit 24, dialregister 25 and line selector circuit '26 may be considered together asa link circuit. For certain embodiments of the system, a synchronizingfrequency may be fed from common equip ment 22 over lead 29 to lineselector circuit 26 and line finder circuit 23 respectively. The fiveleads 2?, 2-8, 29, 3'1 and 38 from common equipment '22 may also bemultipled to other link circuits of the system as shown.

The distributor function of common equipment 22 may be performed by a.rotating distributor in the form of a cathode ray tube as illustrated indetail in Figs. 2 and 3. The distributor tube is indicated generally at39 and may comprise a cathode it, the usual grid 4|, focus and anodeelectrode :32, horizontal deflector plates 43 and vertical deflectorplates 44. Two-phase distributor currents from a suitable sweep controlmay beapplied over leads 45, 45, 47 and '58 to the horizontal andvertical deflector plates respectively, so as to produce a cyclicrotation of the electron beam. At the target end of tube 39 are providedtwenty coupling targets 49 to 68, respectively, which are coupled withthe individual lines I to 20 inclusive. These targets may comprisesecondary electron emissive elements associated with a common anode 69to provide dynodes all having "a common output. A mask or screen is maybe provided, if desired, having apertures therein so that the electronbeam will impinge on each dynode only when the beam is aligned therewiththus preventing possible secondary emission from others. The output ofthe distributor tube 39 is connected from anode 69 over lead ll, thensignal isolating circuits hereafter described to leads 21 and 28 whichgo to the line finder circuit as shown in Fig. 1. The output from theline selecting circuit 26 may be applied as indicated over line 36 tothe grid 41 serving to modulate the beam in accordance with the selectedsignal energy. Thus, referring to Fig. 1, the output from lead H may beapplied after suitable delay (produced in line selecting equipment 26 ashereafter described) over lead 36 to gridII I to provide the desiredcommunication channel between the chosen pair of lines.

The common equipment 22 is illustrated in Fig. 4. For illustrativepurposes a base frequency of 10,000 cycles per second has been selectedas the scanning rate of the rotating distributor. This frequency issufficiently high to reproduce voice frequencies with adequate fidelityfor transmission of speech. For the twenty-line system the basefrequency is derived from a 200 kilocycle stable oscillator I2preferably crystal controlled. This higher frequency is preferablyutilized since it is generally easier to build a more stable oscillatorat the higher frequencies than at the lower 10,000 cycle frequency whichis to be used. Furthermore, in certain of the modifications illustrated,the 200 kilocycle wave may be utilized for other control purposes. Thesinusoidal frequency generated in master oscillator 12 is reduced to thebase frequency of ten kilocycles in frequency divider I3.

The output of frequency divider 13 is applied over 90 phase shifter I4to the vertical and horizontal sets of deflecting plates 43 and 44 ofdistributor tube 39 herein diagrammatically illustrated. This will serveto rotate the beam at a frequency of 10,000 revolutions per second sothat each of the dynodes 49 to 68, illustrated in Figs. 2 and 3 and inthis figure, will be scanned once every 10,000ths of a second. Incominglines I, 5 and are shown connected to the respective dynodes 49, 53 and68.

At 2| is illustrated a typical subscriber sub-set (shown connected toline 5) for use in the system according to our invention. Such a sub-setwill be connected to each of the incoming lines I to 20 inclusive. Thevoice transmitter I5 is connected in series with dial I6 and thenormally open switch hook TI. The receiver I8 is bridged permanentlyacross the line, since, for simplicity of illustration, no separateringing equipment has been illustrated. Accordingly, the signal forsumm'oning a called subscriber may be applied as a special tone whichwill be reproduced in receiver I8 to call the listener to the phone.

As in the usual equipment, switch hook I1 is normally open. However,upon initiating a call, the switch becomes closed, completing a circuitin the calling line loop over low-pass filter I9 and the associatedlines at the sub-set, applying a negative potential from battery 80 tothe associated dynode 49. Normally the dynode electrodes "49 to 68 areat the same potential as anode 68 so no current flows. This negativepotential will produce a difference in potential and cause secondaryemission current to flow from the dynodes upon impingement of the beamof tube 39 thereon, producing a negative output pulse in output line H.The pulses are preferably signal modulated to a depth of only to 50 percent so that there will always be sumcient amplitude to furnish energyto establish and maintain connections regardless of modulating signals.The negative pulses resulting from operation of the selected dynode 49are fed to the grid of inverter tube 8I. The anode circuit of tube 8| iscoupled to the grid of clipper tube 82 which serves to clip these pulsesat a predetermined level to pass only the modulated portions of theincoming pulses. Thus, the output of this tube, representing the speechsignals, may be substantially 100 per cent modulated. Theseclippedpulses are then applied to a cathode follower tube 83 and from there toall of the link circuits over the cathode follower output lead 28. Asecond output is taken across the cathode resistance of inverter tube8|, these pulses being applied to a clipper tube 84 which serves to clipthe pulses to a constant level eliminating modulation effects therefrom.The anode circuit of tube 84 is coupled to the grid of a cathodefollower tube 85 which serves to apply pulses 86 through common feedresistor 81 over wire 21 to the grid of line finder gate tube 88 (shownin Fig. 6) of line finder 23 (shown inFigs. 6 and 1) in the first linkcircuit (now underconsideration) and in parallel to the grids of thecorresponding line finder gate tubes in all other link-s. The pulse 86after passing through resistor 81 may be called 89, so that the pulseactually arriving at the grid of tube 88 and of the other similar tubesis pulse 89. Under the conditions now assumed, when none of the grids ofthe line finder gate tubes is drawing grid current, pulse 89 is nearlyas strong as pulse 86; but under other conditions it may be much weakerthan 86 as hereafter explained. In the absence of any signals on thecathode of this line finder gate tube 88, the above traced pulse 89 onits grid is insufficient to cause the flow of plate current, because ofthe bias applied to the grid is sufficiently far below cutoff.

In the line finder 23 (Figs. 1 and 6) is provided an oscillator 90normally operating at a fre-'- quency slightly lower than the outputfrequencyfrom frequency divider I3 in Fig. 4. This oscillator may, forexample, operate at one-fiftieth of one per cent below the frequency ofthe frequency divider. The output energy from oscillator 90 is appliedto a clipper amplifier 9| which serves to produce rectangular selectingpulses. These pulses are differentiated in a differentiating networkconsisting of condenser 92 and re-,

sister 93, to produce the pulse formation 94 which is applied to thecontrol grid of clipper tube 95. The output pulses 96 from tube(corresponding to the leading edge of pulse 90 and the posi-.

tion part of formation 94) are applied to cathode follower tube 91. Theresulting pulses 98 are applied to the cathode of tube 88 normallytending to make the cathode of this tube more negative so that the tubewill be more nearly conductive. However, except when the pulses 98applied to the cathode of tube 88 coincide with the previously tracedincoming pulses 88, applied via wire 21 to the grid thereof, tube 88 isineffective. Sufiicient bias is applied to the grid of tube 88 frombattery 99 so that it requires the combined amplitudes of the two pulses89 and 98 to operate this tube. As oscillator 90 continues to driftrelative to the output of frequency divider 13, the pulses 98 willcommence to coincide with the pulses 89 incoming from the calling line,overcoming the bias in tube 88 and producing output pulses I00 in line32. These output pulses I00 then are applied over condenser IN to apeaked amplifier and phase corrector circuit I02 which serves to lockoscillator 90 into step with the incoming pulses 89 so that its outputis in synchronism with the frequency from divider I3, and pulses 98 willthen continue to coincide regularly with the incoming pulses 89 from thepredetermined calling line. As soon as the oscillator is locked intostep, the pulses from line 32 also are applied over rectifier I03 and anintegrating network I04 to a control grid of delayed gain control tubeI05. Operation of tube I05 increases the positive voltage on the screenof clipper tube 95 increasing the amplitude of the output pulses 96 andhence 98. The value of resistor 81 and the a grid currentcharacteristics of tube 88 are such that the total positive swing of itsgrid with respect to its cathode cannot exceed a predetermined smallamplitude regardless of :the magnitndes of pulses '98 and 86 which areapplied respectively to the cathode and via resistor 87 to the grid oftube '88. However, the square pulses 38 from tube 91 will increase inamplitude with the change in bias of tube 95. Thus, since the sum ofpulses 89 and .93 roughly constant, while the value of the component 98is rising, it is clear, that the magnitude of pulses 89 must-becorrespondingly decreasing. This decrease in amplitude of pulse 89 iseffective to prevent other line finder gate tubes (similar to 88 but inother links) from responding .as more fully explained hereafterinconjunction with Fig. 10.

This decrease in pulse 89 does not, however, reduce the response of tube88 in the first link (now :under consideration) since the "total :inputbetween grid and cathode is not decreased. Thus, pulses :IOII .areroughly constant in amplitude. These pulses I01! from the line findergate tube 88 are applied also over line '32 and coupling circuit I 06 togate control tube I! which serves to control the suppressor bias on theinput gate tube I08. Tube I08 is normally conditioned by suppressor gridbias so that the pulses applied thereto from the output of cathodefollower 83 over line v28 will not be passed by the tube. However, uponoperation of tube [81, by selection of a predetermined incoming line asdescribed above, the suppressor grid'of tube I 88 has applied to it sucha potential that the tube becomes con-- ductive during the instantscorresponding to the time-channel of such predetermined line.Accordingly then, combined dial-and-speech pulses I09 will be appliedfrom the output of tube IIlBcver line 33 to the pulse forming equipment24 of Figs. 1 and '5 and to the line selecting equipment 26 of Figs. 1and 8. However, theenergy applied to the line selecting equipment ofFig. 8 willnot be passed until such time as line selection has beenveffected which will be described later.

Line finder 23 having now operated, pulses I89 from fine 33corresponding to the time channel-- individual to the predetermined lineassumed to. be calling are applied to an integrating network H0 whichmay or may not be preceded by a pulse stretching circuit similar to apeak voltmeter. These pulses are then amplified in tube III and and areapplied over transformer H2 to the control grid of the clipper tube I I3and to the control grid of a second tube H4. The integrating network H0in the input circuit of tube HI functions as a low-pass filter whichwill pass the dial pulses but will not pass the higher frequencycommunication signals. The clipper H3 serves to shape and clip theincoming dial pulses. to form square wave pulses H5 which in turn aredifierentiated in network H6 and applied to the control grid of dialgate tube H1. Tube H1 is biased so as to suppress the negative part ofthe difierentiated pulse (corresponding to the leading edge of thesquare dial pulse I I5) and to pass only the positive part of thedilferentiated pulse, corresponding to the trailing edge of such squarewave pulse I I5. Normally tube II! is nearly cut-off by the voltage dropin its screen grid resistor H8 which is common with the plate of anormally conducting tube H9 of a flip-flop circuit which operates inconjunction with tube If}. This cut-off bias serves to prevent lowamplitude signals which may precede the dialing pulses from passing andfalsely operating the pulse registcr. Time constants of this filter areso adjusted that the leading edge of the first'dial pulse serves tocause tube H4 to operate, cutting on tube H9. Low-pass filter and timeconstant circuit I20 "in the grid circuit of tube H'9 causes thiscondition to be maintained for the interval of the pulse signal seriesuntil shortl after "the last pulse has passed, when the flip-flopcircuit will return to normal, again rendering the dial gate' tube HIinsensitive. By provision of this special blocking circuit, transienteffects before and after dialing will not afiect the register. Theoutput pulses from dial gate tube H! are applied over line 35 to thedial pulse register circuits 25 of Fig 1, this pulse passing throughresistors I2I and I22 to grids of the first register stage.

The dial pulse register circuits consist of a series of tubes of whichI23, I24, I25 and I26 are shown in detail connected as conventionaltrigger circuits for operation as a binary counter. Blocks I21, I28 andI29 constitute further register trigger circuits not shown in detail,there being a sufficient number of these register circuits to count anydialing number in the exchange. With the system shown for twenty linesthe five shown are sufficient. Initially, the tubes on the right handside such-as I24 and I26 are conducting serving to bias tubes I23 andI25 to cut-0th Furthermore, voltages developed in the register' circuitsare applied as will be described later in: more detail over linesI30-I39 to bias the various delay gate tubes to cut-off and the'zero.gate tubes to conduction in the line selecting circuit of Fig. 8.

The negative pulses incoming over line 35 are applied to the firstregister circuit including tubes I23 and I24. When the register circuitis in its normal condition, that is with tube I24 conducting and tubeI23 biased to cut-off, voltage is applied to line I38 maintaining theassociated zero device of Fig. 8 in operation and over line I31 blockinga delay gate to be described in more de- The first incoming pulse online 35 tail later. passes through resistance I 2| to the grid of tubeI24 thus causing this tube to cut-off rendering, however, tube I23operative and applying control voltages to lines I36 and I3I which serveto block the first zero gate and open the first delay gate. 1

The output from tube I24 is applied over a line- I40 to the secondregister circuit comprisingtubes I25 and I26 serving to transferconduction from tube I26 to I25 and from I25 to I 26' alternately eachtime the trigger circuit I23, I24 re stores to normal condition (i. e.each time tube It will thus be clear" I24 becomes conductive). that thesecond register shifts its condition for every second pulse applied tothe first register while the first register changes its condition forevery incoming pulse.

the second register circuit restores to normal (i. e. eachtimetube I23becomes conductive) making register I21 shift its condition once forevery two The operations of the second register circuit. 7 fourthregister I28 is similarly caused to shift its condition each time thethird register I21 restores to normal and the fifth register I29 issimilarly controlled from the output of the fourth" register I28.

The third register IZ-I-is similarly controlled over line I 4| so thatthe reg ister circuit I 2-! changes its condition each time as I6, Fig.4, for each line are numbered with digits from 1 to 20 representing thetwenty lines. Each dial for any particular line is set so that when acalled line is dialed, a number of pulses corresponding to thedifference between the calling line and the called line will betransmitted to the exchange. It thus becomes necessary to produce timedisplacements in the communication energy corresponding to thedifference in timing between the scanning of the two lines in thecathode ray scanning circuit 39. The difierent signalling pulses operatethrough the pulse register circuit of Fig. 7 as described above, toselect the desired time displacement in accordance with the line whichis being called. To this end, each of the register circuits is providedwith a zero gate I42, I43, I44, I45 and I46 associated with the first,second, third, fourth and fifth register circuits respectively.Likewise, associated with each of these respective registers aredifferent delay gates I48 microseconds for the twenty line system), I49(10 microseconds), I50 (20 microseconds), I5I (40 microseconds) and I52(80 microseconds). Each of these delay gates includes a delay line. Inthe output of each of these delay lines are delay gate tubes I53 and I54being ilustrated in the case of gates I48 and I49. It is understood thatsimilar delay lines and gate tubes are provided for the other delay gatecircuits. In the normal condition, before any pulse arrives, the systemis biased so that the zero gates I42 to I46 are all operative so that nodelay will be provided in any of the pulses I09 incoming over line 33from the line finder circuit of Fig. 6. These pulses I09 therefore willbe applied directly from line 33 through the zero gate circuits I42 toI46 inclusive, and from there over line I55 to the output gate tube I56.Assuming for the moment that tube I56 is not disabled, its platedelivers corresponding pulses I51 over line 36 to the control electrodeof tube 39, Fig. 4, and thence back onto the calling line. The firsttime the first register operates, the control potential is transferredfrom line I30 to line I3I rendering tube I53 conductive and biasing tubeI42 to cutoff. Thus, if one pulse only is dialed, a delay of fivemicroseconds is produced so that the energy incoming over line 33 willpass through the first delay gate I48 and the remaining zero gates I43to I46 inclusive. The second pulse transfers the control potential fromline I3I back to I30 causing zero gate I42 again to become operative andblocking tube I53 in delay gate I48. At the same time, the secondregister operates transferring the potential from line I32 to line I33blocking the second zero gate I43 and opening gate tube I54 in thesecond delay gate I49 introducing a ten microsecond delay between line33 and line I55. Thus, the second pulse will produce zero delay in I42,ten microsecond delay in I49 and zero delays in I 44 to I46. The thirdincoming pulse will not affect the second register circuit but willagain operate the first register circuit introducing the fivemicrosecond delay gate I48 as well as the ten microsecond delay gate I49producing a fifteen microsecond delay in the incoming energy. The fourthpulse then will return both the first and second register to normal butwill operate the third register I2'I producing a twenty microseconddelay at delay gate I50. The fifth pulse will again insert the fivemicrosecond delay gate I 48 so that there will be five and twentymicrosecond delays producing a total of twenty-five microseconds. Thenext pulse will switch out the five microsecond delay line and switch inthe ten microsecond delay line producing a total delay of thirtymicroseconds. The next pulse will switch in the five microsecond delayline while leaving the ten and twenty microsecond delay ineffective thusproducing thirty-five microsecond delay. The next successive pulse willthen render delay lines I48, I49 and I50 ineffective but will bring intocircuit the fourth delay gate I 5| with its forty micro-,

second delay. The other pulses will then bring in, in similar sequence,the five, ten and twenty microsecond delay gates I48, I 49 and I50introducing in sequence five microsecond delays until delay gate I52 isoperated whereupon theprocess will again be repeated in five microsecondsteps. Thus, with the five delay gates it is possible to produce anydesired delay condition in the twenty iines. It will be clear that if adifferent number of lines are provided, additional stages for the binarycounting system and additional zero gates and delay gates similar tothose outlined herein may be provided to secure the proper delay ininterconnection for any number of lines.

After the desired number has been dialed, the signalling energy from thecalling subscriber Will be transmitted as described over the commonequipment circuit and line 33 in the link circuit to the grid of tube I56. The output pulse I5'I from tube I56 is then transferred over line 36to the control electrode of tube 39 as illustrated. The voicemodulations of pulses I51 incoming over line 36 will then producevariations in the electron stream of tube 39 each time the beam isaligned with the called line electrode and this variation in energy willbe passed over the line to the corresponding low-pass filter I9 of thecalled subscriber to the receiver circuit I8. For the purpose ofcalling, a tone frequency may be transmitted to operate any suitabletone control apparatus at the called subscribers line or the output ofreceiver 18 may be such that attention is directed to the phone directlyby whistle or other call transmitted by the calling subscriber.

In the foregoing it has been assumed that tube I56 was conducting, forthe purpose of simplicity of explanation. Actually this tube is normallybiased to cut-off in order that the dialing pulses incoming over linkcircuit 23 do not affect other lines during the dialing. This cut-offbias of output gate tube I56 is controlled by the gate control circuitcomprising tubes I58 and I59. Tube I58 is normally conductingmaintaining the grid of tube I56 biased to cut-01f. These tubes I58, I59in turn are controlled by tube II9 as follows: As explained above tubeII9 of Fig. 5 becomes cut-off at the beginning of a series of dialpulses. At such time it sends out an ineffective positive pulse throughcondenser I69 to the grid of tube I58. As soon as the dialing operationis complete, however, tube II9 returns to conducting condition sendingout a negative pulse. This negative pulse cuts off tube I58, which inturn renders tube I59, and also gate tube I56, conductive. This permitsthe message energy to be transferred over line 36 to the calledsubscribers line.

In order to protect the called line from being seized by the linefinders of other links when the called subscribers receiver is removedfrom the hook, a portion of the delayed pulse I5! is tapped from line 36over line 31 through isolating resistors I6I in Fig. 4 to a busy pulseshaper I62 from whence it is .conducted to the grid of busy gate tubeI63. This limits the maximum possible value of the positive line finderpulse 89 from tube which will be applied, after the called sub- 13 tubeI58 conducting. Thus, the whole link circuit is restored to normal.

In order that the pulses from any one incoming linemay be effectivelyreduced in amplitude so as to prevent other line finders from thereafterseizing the same calling line I, the delayed gain tube I and associatedcircuit are provided. It will be clear from the above description thatwhen two or more subscribers are using the exchange at the same timethere will be a plurality of differently timed pulses in the linecircuits of the common equipment of Fig. 4. These pulses from the outputof cathode follower 83 are applied to all of the link circuits inparallel. When one link circuit, however, has taken hold it is necessarythat the pulses of this selected circuit be made inefiective to seizeother links. A better understanding of the operation of the system toprevent this operation may be had by reference to Figs. 4 and 5 and thecurves illustrated in Fig. 10. a

The pulses from the anode 69 of tube 39 are applied to the grid of tube8! which has separate late and cathode outputs. The pulses from theplate output of tube 8I varying in amplitude in accordance with anincoming signal are shown are passed out through the plate circuit ofthis tube to cathode follower 83. Preferably, the

energy is only about 25% modulated so that the modulation variationswill constitute the minor portion of the pulsing energy. These pulsesare used for transmitting speech and are not of interest in connectionwith the feature now being considered.

The pulses from the cathode output of tube 8| are the ones of primaryinterest. These pulses are clipped in tube 84 and passed through cathodefollower 85 so as to produce a series of equal amplitude pulses 86 asshown in curve IIlB. These pulses 86 are applied through resistors 81 aspulses 89 to the grids of all line finder gate tubes 88 in Fig. 6.Lock-in oscillator 90 produces an output wave I12, curve IIlC, whoseperiod is slightly longer than the time interval between two pulses 89.Wave I12 is clipped :at clipping levels I13 and I14 then differentiatedand again clipped to produce pulses whose leading edges substantiallycoincide with the instant of rise of wave I12 between the clippinglevels. These pulses which are preferably substantially wider than theincoming pulses 89, pass through cathode follower 91 and the resultingpulses 98 are applied to the gate tube 88. Since the frequencies areslightly different, the phase or time position of pulses 89 willcontinually shift with respect to pulses 98 until pulse 89 coincideswith pulse 98 as shown in curve IUD. When this occurs, the line findergate 88, Fig. 6, is operated so that the pulses may pass through peakedamplifier I02 to the oscillator 98 locking it into step with the pulses.The phase correction of peaked amplifier I92 is so adjusted that sinewave I12 will rise through zero slightly before the time of arrival ofpulse 89. The pulses 98 will then be produced in fixed time relationshipwith pulses 89 as shown in first waveform of curve IOE. Once thesepulses are synchronized, the delay gain tube I05 cuts off increasing thescreen bias of tube 95 so that the selecting pulses 98 increase fromtheir normal search" amplitude to a much higher holding amplitude asshown in the second wave form in curve HIE, thus reducing the effectiveheight of 4.. pulses 89. Thus, pulses 89 applied to the grids of theline finder gate tubes (corresponding to tube 88) in all other linefinders will be very small as shown in-the third waveform of curve 18E.Then even if coincidence between these pulses 89 and the normal orsearch selecting pulse 98 of such other line findersdoes occur, nosignal will be passed through the gate tubes of such other line findersas shown in the fourth waveform of curve IllE.

When the called party answers, the closure of I his line loop 5 placeson the dynode 53 a potential similar to that of a calling line. If nospecial precautions were taken this would cause another line finder toseize the called partys line thus tying up an additional link. To avoidthis, the busy shaper I62, and busy gate I63 are provided which functionas follows:

After the completion of dialing the outputgate,

ergy of these pulses I51 is branched from line 38 in Fig. 8 and passesover line 31 and isolating resistor I 9| to the busy gate shaper I62,which amplifies, clips and reshapes these pulses into strong, sharpconstant amplitude pulses. (For this purpose the clipping level ofspeech clipper tube 82 should be set so that the speech modulation neverreduces pulses I1I below a small fixed minimum value.) The reshapedpulses from I82 are applied to the grid of busy gate tube I63 to makethis momentarily highly conductive. This gate tube I63 then imposes afixed upper limit upon the amplitude of the positive pulses 89, so

" that these cannot attain an amplitude sufficient to cause seizure ofthe called line by another line finder. Preferably, however, this upperlimit is high enough to hold a line finder which has already lockeditself to the called line (in order that the act of selecting a linealready engaged as calling line in a previous connection shall not breakdown such previous connection).

Turning to Fig. 11, there is illustrated a delay line in the systemwhere the longer delays are required. For the shorter intervals shown indelay gates I48, I49 and I58 of five, ten and twenty microseconds,artificial delay lines of known form may readily be used. However, forthe longer delays, acoustic delay means may be preferable. The line may,for example, comprise a container I15 filled with mercury I16, having alength where V is the velocity of sound in the liquid and D is thedesired delay time. At the input end is provided a crystal, for examplea quartz I11, in a suitable mounting ring I18, with an electrode I19coupled with line I89 for the input signal.

While we have disclosed a particular embodiment of our invention and aparticular telephone exchange equipment in which this embodiment isused, it should be distinctly understood that many modifications andapplications of our invention other than those shown in the specificexamples may occur to those skilled in the art. The particularembodiment described herein is given merely by way of example and is notto be considered as any limitation on the scope of our invention as setforth in the objects thereof and in the appended claims.

is we claim:

1-. 'Ina communication exchange system, a p1u-- means for producingdiscrete pulses having per-- tl'ens "varying in amplitude inaccordancewith signals; means including a normally ineffective line finder circuitfor" connecting a line to an outgoing circuit, means for using a portionof said pulses below said varying portion to operate and maintainefiective said line finder cir--- cult; and a circuit to-carr saidsignals to'said outgoing circuit by" said varying portions.

The communication exchange system according to claim 1", and in whichthe unm'odulated portions of said pulses areof' constant 'amplitude andoperate and maintain eifective the line finder circuit, and themodulated portions carry the signals to said outgoing circuit.-

3. In lei-communication exchange system, a" pluralit'y of channels;means for applying signals to the channels, means for successivelyscanning said channels, means for placing in signalling condition one ofsaid channels, means operative thereupon for applying a voltage of apredetermined level to said one channel whereby pulses are produced whensaid channel is scanned, the signals applied to said channel modulatesaid pulses and are' below said voltage level, a multielectrodedi'schargedevice having two output circuits, the first outputcireuit-conditionedto pass portions-of saidpulses at a substantiallyconstant amplitude level below said signals; and the second outputcircuit conditioned to pass portions of said: pulses above-said constantamplitude level, anetwork terminating said channels for applying theproduced pulses to-said discharge device, means including a linefinderresponsive to said pulses the'first output circuit" for establishing 5';The system according to claim 3; and in" Wlilchthe signals applied tosaid channel are'not greater than fifty percent of said voitagelevel;

6; The system according to claim 3, and in which'the dischargedevice-has an anodein one and a" cathode in the other output circuit,and

a gi i'cl for said discharge device connected with said network.

"7i The system according to claim 6; and in" whichthe cathodeisconnected withthe-fi'ist and the'j anode with the" second outputcircuit.

8. The system accordin to claim 6, and in" which the line finderresponds to the output cif= cuit connected with the cathode.-

EATJ'LL. R. ADAMS; DAVID HZ RANSOM;

REFERENCES CITED The following references are of record-inthe file ofthis patent:

UNITED STATES PATENTS

